1. Field of the Invention
The present invention relates to a transmitter system that detects physical quantities, converts the physical quantities into electrical signals, and allows the signals to be transmitted to a load through a transmission line. More particularly, the invention relates to a transmitter system that generates a 4-20 mA two-wire standardized current output.
2. Description of the Prior Art
FIG. 1 is a schematic view illustrating the configuration of a conventional transmitter system. In the figure, transmitter 10 is formed according to a two-wire method and is connected to transmission line 19.
Transmission line 19 is connected to a series circuit of instrumentation power supply E1 and load R1. Terminal 12 is connected across load R1. Load R1 is assumed to be, for example, 250 Ω.
In the example of the conventional transmitter system configured as discussed above and illustrated in FIG. 1, the output of transmitter 10 is a voltage change in load R1 and the change is sent to terminal 12.
FIG. 2 is a block diagram illustrating the configuration of a conventional transmitter and the configuration is described hereinafter.
Sensor 21 detects physical quantities, for example, differential pressure, static pressure, capsule temperature and amplifier temperature.
Conversion block 22 is connected to sensor 21 and converts the output thereof into electrical signals. In addition, conversion block 22 is internally equipped with a switching means (not shown in the figure) for selecting differential pressure, static pressure or temperature.
Microprocessor (MPU) 23 is connected to conversion block 22 and performs such tasks as corrective calculation and communication for the output of the conversion block.
Gate array (G/A) 24 is connected to MPU 23 and performs pulse width modulation (PWM) on the output of MPU 23.
Digital-to-analog conversion block (D/A conversion block) 25 is connected to G/A 24 and generates a 4-20 mA two-wire standardized current output according to electrical signal PWM1 from G/A 24 and sends the output to instrumentation power supply E1.
Shunt regulator 26 is supplied with power from instrumentation power supply E1 and supplies voltage V1 to conversion block 22, series regulator 27, D/A conversion block 25, and so on. Voltage V1 is assumed to be, for example, 5 V.
Series regulator 27 is supplied with power from shunt regulator 26 and supplies voltage V2 to MPU 23, G/A 24, and so on. Voltage V2 is assumed to be, for example, 3 V. In addition, series regulator 27 is equipped with zener diode D3 whose zener voltage is 2.5 V.
Now, an explanation will be made to how the example of the conventional transmitter system illustrated in FIG. 2 operates.
First, in order to measure differential pressure, the switching means within conversion block 22 selects the option “differential pressure.” The differential pressure to be measured is detected by sensor 21, converted into an electrical signal by conversion block 22, calculated correctively and communicated by MPU 23, pulse-modulated by G/A 24, converted into a 4-20 mA two-wire standardized current output by D/A conversion block 25, and sent to instrumentation power supply E1.
Second, in order to measure static pressure, the switching means within conversion block 22 selects the option “static pressure.” The static pressure to be measured is detected by sensor 21, converted into an electrical signal by conversion block 22, calculated correctively and communicated by MPU 23, pulse-modulated by G/A 24, converted into a 4-20 mA two-wire standardized current output by D/A conversion block 25, and sent to instrumentation power supply E1.
Likewise, in order to measure temperature, the switching means within conversion block 22 selects the option “temperature.” In this way, the transmitter selects from the options “differential pressure,” “static pressure,” “temperature” and so on according to the object of measurement, and outputs their values.
Some conventional transmitters are designed to optionally select from differential pressure, static pressure and temperature signals (see patent document 1, for example).                Patent document 1: Patent gazette 2644742        
However, in order to simultaneously transmit differential pressure, static pressure, temperature and other physical quantities using the example of the conventional transmitter system or transmitter illustrated in FIG. 1 or FIG. 2, it is necessary to configure a transmitter system by combining two or more transmitters. As a result, the problem is that the system becomes large and expensive.